In a once-through steam generator, heating a plurality of steam generator tubes which collectively form an evaporator heating surface leads to a complete evaporation of a flow medium in the steam generator tubes in one pass. Prior to its evaporation the flow medium—typically water—is in this case normally supplied to a preheater, usually also referred to as an economizer, which is connected upstream of the evaporator heating surface on the flow medium side, where it is preheated.
The feedwater mass flow into the evaporator heating surface is regulated as a function of the operating state of the once-through steam generator and, in connection therewith, of the current steam generator output. In the event of load changes the evaporator throughflow rate should be varied as far as possible in synchronism with the heat input into the evaporator heating surface, because otherwise it is not possible to avoid with certainty a deviation of the specific enthalpy of the flow medium at the outlet of the evaporator heating surface from the setpoint value. Such an undesirable deviation of the specific enthalpy makes it more difficult to regulate the temperature of the live steam being discharged from the steam generator and furthermore leads to high material stresses and consequently to a reduced useful life of the steam generator.
In order to keep deviations of the specific enthalpy from the setpoint value and undesirably large fluctuations in temperature resulting therefrom in all operating states of the steam generator, i.e. in particular also in transient states or during load changes, as small as possible, the feedwater throughflow rate adjustment can be embodied in the manner of what is termed a predictive or anticipatory implementation. In this case the necessary feedwater setpoint values should be provided in particular also in the case of load changes as a function of the current operating state or the operating state that is to be expected in the immediate future.
EP 0 639 253 discloses a once-through steam generator in which the feedwater throughflow rate is regulated by way of a precalculation of the required volume of feedwater. Serving as a basis for the calculation method in this case is the heat flow balance of the evaporator heating surface, into which the feedwater mass flow should enter, in particular at the inlet of the evaporator heating surface. The setpoint value for the feedwater mass flow is in this case specified from the ratio of the heat flow currently transferred from the heating gas to the flow medium in the evaporator heating surface on the one hand and a setpoint enthalpy increase of the flow medium in the evaporator heating surface specified with regard to the desired live steam state on the other hand.
In real-world situations, however, measuring the feedwater mass flow directly at the inlet of the evaporator heating surface has proven technically complex and cannot be performed reliably in every operating state. Instead of this the feedwater mass flow is alternatively measured at the inlet of the preheater and incorporated into the calculations of the feedwater volume, though said feedwater mass flow is not equal in every case to the feedwater mass flow at the inlet to the evaporator heating surface.
In order to counteract the inaccuracies caused thereby in the specification of a setpoint value for the feedwater mass flow that is particularly appropriate to demand, in particular when load changes occur, it is provided in an alternative concept of a predictive mass flow regulation as known from WO 2006/005708 A1 to take into account the feedwater density at the inlet to the preheater as one of the input variables for regulating the feedwater throughflow rate.
Both of the cited concepts for predictive mass flow regulation are based on the setpoint value for the steam generator output as the principal input variable, from which the characteristic values being incorporated into the actual determination of the setpoint value are calculated on the basis of stored correlations and in particular with recourse to previously obtained calibration or reference measurements. This, however, presupposes sufficiently stable system characteristics overall that are unequivocally attributable to a firing capacity, as are typically present in fired steam generators. However, conditions of this kind are not present in other systems, such as for example in an embodiment of the once-through steam generator as a heat recovery boiler for recovering heat from the exhaust gas of an upstream gas turbine. Moreover, in these types of systems connected as heat recovery boilers a firing capacity cannot be used to the same extent as a free parameter as in directly fired boilers, since in the case of a connection as a heat recovery boiler the operation of the gas turbine is usually regarded as the primary criterion for controlling the overall installation, with the other components being adjusted to bring them into line with the system state of said gas turbine.
In order to take account of this knowledge, a further improved predictive mass flow regulation for a once-through heat recovery steam generator is known from EP 2 065 641. The concept disclosed therein provides a precontrolled calculation of the feedwater volume by way of heat flow balancing of the evaporator, preferably including the superheater heating surfaces connected upstream on the exhaust gas side. This means that under favorable conditions for the available heat present on the exhaust gas side an evaporator throughflow rate that is adjusted to demand can be set at all times. For smaller corrections of the feedwater mass flow an overlaid and slow-acting enthalpy controller is provided in this case.